1. Field of the Invention
The present invention relates to a self-propelled construction machine, in particular a road milling machine, a recycler or a surface miner, comprising a machine frame supported by a chassis that comprises front and rear crawler tracks or wheels.
2. Description of the Prior Art
The known construction machines comprise a working device that is arranged on the machine frame and comprises a working roller for working the ground. The working roller can be a milling roller and/or a cutting roller. Road milling machines, for example, comprise a milling roller which can be used to mill away damaged road surfaces.
The individual crawler tracks or wheels of the construction machine are associated with lifting devices that can each be retracted or extended with respect to the machine frame so that the machine frame is lowered or raised, respectively, with respect to the ground surface together with the working device. As a result, the milling depth in a road milling machine can be adjusted. To adjust the milling depth, road milling machines comprise a control unit, by which the lifting devices are actuated to obtain the desired milling profile.
While the ground is being worked, it is desirable for the machine frame, together with the working device, to be exactly level with respect to the ground surface. To adjust the milling depth, the lifting devices can be retracted or extended. During the milling operation, the working roller of the working device is engaged with the ground.
WO 2006/094795 A1 (U.S. Pat. No. 7,828,309) describes a construction machine in which the lifting devices are positively coupled in such a way that the left front crawler track and the right rear crawler track can be adjusted in height in the same direction and can be adjusted in height in the opposite direction towards the right front crawler track and the left rear crawler track. As a result, the transverse and longitudinal tilt of the construction machine is reduced when the machine travels over obstacles, and the stability of the construction machine is increased.
WO 2008/077963 A1 (U.S. Pat. No. 8,424,972) describes a road milling machine comprising a control unit by which the lifting devices are actuated during the milling process in such a way that the machine frame is level in parallel with the ground surface. A road milling machine comprising a controller for levelling the machine frame in parallel with respect to the ground during the milling process is also known from US 2013/0162004 A1.
EP 2 722 441 A1 (U.S. Pat. No. 8,998,344) discloses a road milling machine comprising a control unit for the lifting devices for preventing sideways tipping movements of the construction machine. During the milling process, the control unit is activated such that the milling roller is precisely aligned for the desired milling profile.
Owing to the predefined working width of the working device, relatively large areas can only be worked by the self-propelled construction machines in portions, and so the construction machine has to be moved to the next portion once a previous portion has been worked. It may also be necessary to move the construction machine to a new site in order to be transported. To work the ground, the working roller has to be engaged with the ground, whereas, in order to prevent damage to the machine or to avoid the ground being worked accidentally, the working roller should not be in contact with the ground while the construction machine is being moved. When the construction machine is moved, the individual crawler tracks or wheels cross transitions between milled and unmilled terrain portions. If sudden transitions have to be crossed, the machine frame may tilt sharply to one side.
EP 1 860 241 A2 (U.S. Pat. No. 7,530,641) describes a controller for a road milling machine, which controller is not activated during the milling process of the construction machine, but rather while the machine is being moved, in order to prevent the milling roller engaging with the ground. The controller monitors the distance between the milling roller and the ground in order to decouple the working device from the drive unit and/or to extend the lifting devices if the distance falls below a minimum distance.
US 2013/0166155 A1 also discloses a controller for the operating state of the construction machine, in which the working device is not operational. This controller adjusts different positions of the machine frame of the construction machine. In one of the positions of the machine frame, all the lifting devices are intended to be extended to the same length. In addition, the machine frame is also intended to be aligned with the ground surface. Therefore, if the ground surface is sloped, the machine frame is slanted too.